[Haskell] Dynamic binding

[Ralf Lammel]

Bulat,

Bulat wrote:

> just create list of draw functions itself:
> 
> [drawCircle (10,10) 5, drawSquare (20,20) 10]

No! the exercise is about lists of shapes
not lists of results of drawing shapes.
This is clearly a major difference.


Bulat wrote:

> for more complex tasks - declare interface as a structure:
> 
> data ShapeInterface = Shape { draw :: IO (),
>                               moveTo :: Point -> IO (),
>                               calcArea :: Float
>                             }

No! You miss the point that the different shapes
differ regarding state types.
You don't have a chance when you use one datatype.

haskell-cafe?

Ralf

> -----Original Message-----
> From: haskell-bounces at haskell.org [mailto:haskell-bounces at haskell.org]
On
> Behalf Of Bulat Ziganshin
> Sent: Thursday, June 23, 2005 12:18 AM
> To: Andrew Ward
> Cc: Pal-Kristian Engstad; haskell at haskell.org
> Subject: Re[2]: [Haskell] Dynamic binding
> 
> Hello Andrew,
> 
> Thursday, June 23, 2005, 5:38:03 AM, you wrote:
> 
> AW> To handle the problem of drawing all shapes, in c++, I would have
a
> list
> AW> of shape pointers:
> 
> AW> struct shape{ virtual void draw(...);};
> AW> struct circle : public shape {...};
> AW> struct square : public shape {...};
> AW> std::list<shape *> shapes;
> AW> for(std::list<shape *>::iterator it = shapes.begin();it !=
> AW> shapes.end();++it)
> { (*it)->>draw(...); }
> 
> AW> This general pattern of dynamic binding I use over and over again.
> Could
> AW> you give me some example code of this type of thing handled in
> Haskell's
> AW> way? Assuming that the number of classes deriving from shape might
get
> AW> quite large.
> 
> just create list of draw functions itself:
> 
> [drawCircle (10,10) 5, drawSquare (20,20) 10]
> 
> you are not expected that this problem can be solved with one line of
> code? :)
> 
> 
> for more complex tasks - declare interface as a structure:
> 
> data ShapeInterface = Shape { draw :: IO (),
>                               moveTo :: Point -> IO (),
>                               calcArea :: Float
>                             }
> 
> 
> and return this structures from "constructor" functions:
> 
> circle x y r = Shape { draw = drawCircle center r,
>                        moveTo newCenter = ....,
>                        calcArea = pi*r*r
>                      }
>                where center = Point x y
> 
> square x y size = Shape { draw = ....,
>                           moveTo newCenter = ....,
>                           calcArea = size*szie
>                         }
> 
> figures = [circle 1 2 3, square 4 5 6, circle 7 8 9]
> 
> 
> 
> of course, you cannot use inherited field names when using this
> technique :)
> 
> --
> Best regards,
>  Bulat                            mailto:bulatz at HotPOP.com
> 
> 
> 
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